Large area deposition of graphene on substrates, and products including the same

ABSTRACT

Certain example embodiments of this invention relate to the use of graphene as a transparent conductive coating (TCC). A substrate having a surface to be coated is provided. A self-assembled monolayer (SAM) template is disposed on the surface to be coated. A precursor comprising a precursor molecule is provided, with the precursor molecule being a polycyclic aromatic hydrocarbon (PAH) and discotic molecule. The precursor is dissolved to form a solution. The solution is applied to the substrate having the SAM template disposed thereon. The precursor molecule is photochemically attached to the SAM template. The substrate is heated to at least 450 degrees C. to form a graphene-inclusive film. Advantageously, the graphene-inclusive film may be provided directly on the substrate, e.g., without the need for a liftoff process.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application incorporates by reference the entire contents of eachof application Ser. Nos. 12/461,343; 12/461,346; 12/461,347; and12/461,349, which each were filed on Aug. 7, 2009.

FIELD OF THE INVENTION

Certain example embodiments of this invention relate to thin filmscomprising graphene. More particularly, certain example embodiments ofthis invention relate to the large area deposition of graphene, directlyor indirectly, on glass and/or other substrates, and/or productsincluding the same. This may be accomplished in certain exampleembodiments via pyrolysis of polycyclic aromatic precursors. Certainexample embodiments of this invention advantageously do not requirelift-off and transfer of a graphene sheet.

BACKGROUND AND SUMMARY OF EXAMPLE EMBODIMENTS OF THE INVENTION

Indium tin oxide (ITO) and fluorine-doped tin oxide (FTO or SnO:F)coatings are widely used as window electrodes in opto-electronicdevices. These transparent conductive oxides (TCOs) have been immenselysuccessful in a variety of applications. Unfortunately, however, the useof ITO and FTO is becoming increasingly problematic for a number ofreasons. Such problems include, for example, the fact that there is alimited amount of the element indium available on Earth, the instabilityof the TCOs in the presence of acids or bases, their susceptibility toion diffusion from ion conducting layers, their limited transparency inthe near infrared region (e.g., power-rich spectrum), high leakagecurrent of FTO devices caused by FTO structure defects, etc. The brittlenature of ITO and its high deposition temperature can also limit itsapplications. In addition, surface asperities in SnO2:F may causeproblematic arcing (in some applications).

Thus, it will be appreciated that there is a need in the art for smoothand patternable electrode materials with good stability, hightransparency, and excellent conductivity.

The search for novel electrode materials with good stability, hightransparency, and excellent conductivity is ongoing. One aspect of thissearch involves identifying viable alternatives to such conventionalTCOs. In this regard, the inventor of the instant invention hasdeveloped a viable transparent conductive coating (TCC) based on carbon,specifically graphene.

The term graphene generally refers to one or more atomic layers ofgraphite, e.g., with a single graphene layer or SGL being extendible upto n-layers of graphite (e.g., where n can be as high as about 10).Graphene's recent discovery and isolation (by cleaving crystallinegraphite) at the University of Manchester comes at a time when the trendin electronics is to reduce the dimensions of the circuit elements tothe nanometer scale. In this respect, graphene has unexpectedly led to anew world of unique opto-electronic properties, not encountered instandard electronic materials. This emerges from the linear dispersionrelation (E vs. k), which gives rise to charge carriers in graphenehaving a zero rest mass and behaving like relativistic particles. Therelativistic-like behavior delocalized electrons moving around carbonatoms results from their interaction with the periodic potential ofgraphene's honeycomb lattice gives rise to new quasi-particles that atlow energies (E<1.2 eV) are accurately described by the(2+1)-dimensional Dirac equation with an effective speed of lightν_(F)≈c/300=10⁶ ms⁻¹. Therefore, the well established techniques ofquantum electrodynamics (QED) (which deals with photons) can be broughtto bear in the study of graphene—with the further advantageous aspectbeing that such effects are amplified in graphene by a factor of 300.For example, the universal coupling constant α is nearly 2 in graphenecompared to 1/137 in vacuum. See K. S. Novoselov, “Electrical FieldEffect in Atomically Thin Carbon Films,” Science, vol. 306, pp. 666-69(2004), the contents of which are hereby incorporated herein.

Despite being only one-atom thick (at a minimum), graphene is chemicallyand thermally stable (although graphene may be surface-oxidized at 300degrees C.), thereby allowing successfully fabricated graphene-baseddevices to withstand ambient conditions. High quality graphene sheetswere first made by micro-mechanical cleavage of bulk graphite. The sametechnique is being fine-tuned to currently provide high-quality graphenecrystallites up to 100 μm² in size. This size is sufficient for mostresearch purposes in micro-electronics. Consequently, most techniquesdeveloped so far, mainly at universities, have focused more on themicroscopic sample, and device preparation and characterization ratherthan scaling up.

Unlike most of the current research trends, to realize the fullpotential of graphene as a possible TCC, large-area deposition of highquality material on substrates (e.g., glass or plastic substrates) isessential. To date, most large-scale graphene production processes relyon exfoliation of bulk graphite using wet-based chemicals and startswith highly ordered pyrolytic graphite (HOPG) and chemical exfoliation.As is known, HOPG is a highly ordered form of pyrolytic graphite with anangular spread of the c axes of less than 1 degree, and usually isproduced by stress annealing at 3300 K. HOPG behaves much like a puremetal in that it is generally reflective and electrically conductive,although brittle and flaky. Graphene produced in this manner is filteredand then adhered to a surface. However, there are drawbacks with theexfoliation process. For example, exfoliated graphene tends to fold andbecome crumpled, exists as small strips and relies on a collage/stitchprocess for deposition, lacks inherent control on the number of graphenelayers, etc. The material so produced is often contaminated byintercalates and, as such, has low grade electronic properties.

An in-depth analysis of the carbon phase diagram shows process windowconditions suitable to produce not only graphite and diamond, but alsoother allotropic forms such as, for example, carbon nano-tubes (CNT).Catalytic deposition of nano-tubes is done from a gas phase attemperatures as high as 1000 degrees C. by a variety of groups.

In contrast with these conventional research areas and conventionaltechniques, the assignee of the instant application has previouslydescribed a scalable technique to hetero-epitaxially grown (HEG)mono-crystalline graphite (n as large as about 15) and convert it tohigh electronic grade (HEG) graphene (n< about 3). See, for example,application Ser. Nos. 12/461,343; 12/461,346; 12/461,347; and12/461,349, each of which is hereby incorporated herein by reference inits entirety. The assignee of the instant application also has describedthe use of HEG graphene in transparent (in terms of both visible andinfrared spectra), conductive ultra-thin graphene films, e.g., as analternative to the ubiquitously employed metal oxides window electrodesfor a variety of applications (including, for example, solid-state solarcells). The previously described growth technique was based on acatalytically driven hetero-epitaxial CVD process which takes place atemperature that is low enough to be glass-friendly. For example,thermodynamic as well as kinetics principles allow HEG graphene films tobe crystallized from the gas phase on a seed catalyst layer at atemperature less than about 700 degrees C.

Certain embodiments in such previous descriptions used atomic hydrogen,which has been proven to be a potent radical for scavenging amorphouscarbonaceous contamination on substrates and being able to do so at lowprocess temperatures. It is also extremely good at removing oxides andother overlayers typically left by etching procedures.

Certain example embodiments of this invention, by contrast, provide forthe large area deposition of graphene, directly or indirectly on glassand/or other substrates. Such techniques may be accomplished viapyrolysis of polycyclic aromatic precursors. More particularly, certainexample embodiments of this invention involve the hetero-epitaxialgrowth of graphene from a supra-molecular species. Advantageously,graphene may be formed on substrates without the need for a liftoffprocess in certain example embodiments.

In certain example embodiments of this invention, a method of making acoated article is provided. A substrate having a surface to be coated isprovided. A self-assembled monolayer (SAM) template is disposed on thesurface to be coated. A precursor comprising a precursor molecule isprovided, with the precursor molecule being a polycyclic aromatichydrocarbon (PAH) and discotic molecule. The precursor is dissolved toform a solution. The solution is applied to the substrate having the SAMtemplate disposed thereon. The precursor molecule is photochemicallyattached to the SAM template. The substrate is slowly heated to at least450 degrees C. (potentially as high as 900 degrees C.) to form agraphene-inclusive film in an atmosphere comprising or consisting ofinert gas and/or hydrocarbons.

In certain example embodiments of this invention, a method of making acoated article is provided. A substrate having a surface to be coated isprovided. A self-assembled monolayer (SAM) template is disposed on thesurface to be coated. A solution is applied to the substrate having theSAM template disposed thereon, with the solution comprising a precursorincluding a precursor molecule, and with the precursor molecule being apolycyclic aromatic hydrocarbon (PAH) molecule. The precursor moleculeis attached to the SAM template by irradiating UV energy thereon. Thesubstrate is heated to at least 450 degrees C. to form agraphene-inclusive film. The SAM template and/or the precursor moleculecomprise(s) one or more alkyl groups to help ensure that the c-axis ofprecursor molecule is substantially perpendicular to substrate prior toand/or following the photochemical attaching.

In certain example embodiments of this invention, a method of making anelectronic device is provided. A substrate having a surface to be coatedis provided. A self-assembled monolayer (SAM) template is disposed onthe surface to be coated. A precursor comprising a precursor molecule isprovided, with the precursor molecule being a polycyclic aromatichydrocarbon (PAH) and discotic molecule. The precursor is dissolved toform a solution. The solution is applied to the substrate having the SAMtemplate disposed thereon. The precursor molecule is photochemicallyattached to the SAM template. The substrate is heated to at least 450degrees C. to form a graphene-inclusive film. The substrate with thegraphene-inclusive film is built into the electronic device.

In certain example embodiments of this invention, a method of making anelectronic device is provided. A substrate having a surface to be coatedis provided. A self-assembled monolayer (SAM) template is disposed onthe surface to be coated. A solution is applied to the substrate havingthe SAM template disposed thereon, with the solution comprising aprecursor including a precursor molecule, and with the precursormolecule being a polycyclic aromatic hydrocarbon (PAH) molecule. Theprecursor molecule is attached to the SAM template by irradiating UVenergy thereon. The substrate is heated to at least 450 degrees C. toform a graphene-inclusive film. The substrate with thegraphene-inclusive film is built into the electronic device. The SAMtemplate and/or the precursor molecule comprise(s) one or more alkylgroups to help ensure that the c-axis of precursor molecule issubstantially perpendicular to substrate prior to and/or following thephotochemical attaching.

In certain example embodiments of this invention, a method of making acoated article is provided. A substrate having a surface to be coated isprovided. A monolayer template is disposed on the surface to be coated.A gaseous stream comprising a carrier gas and a precursor molecule isprovided proximate to the substrate having the monolayer templatedisposed thereon, with the precursor molecule being a polycyclicaromatic hydrocarbon (PAH) molecule. The precursor molecule is attachedto the monolayer template by irradiating UV energy thereon. Thesubstrate with the monolayer template and the precursor molecule isheated to form a graphene-inclusive film. The monolayer template and/orthe precursor molecule comprise(s) one or more alkyl groups to helpensure that the c-axis of precursor molecule is substantiallyperpendicular to substrate prior to and/or following the photochemicalattaching.

The features, aspects, advantages, and example embodiments describedherein may be combined to realize yet further embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages may be better and morecompletely understood by reference to the following detailed descriptionof exemplary illustrative embodiments in conjunction with the drawings,of which:

FIG. 1 is a high level flowchart illustrating the overall techniques ofcertain example embodiments;

FIG. 2 is an example schematic view of the catalytic growth techniquesof certain example embodiments, illustrating the introduction of thehydrocarbon gas, the carbon dissolving, and the possible results ofquenching, in accordance with certain example embodiments;

FIG. 3 is a flowchart illustrating a first example technique for dopinggraphene in accordance with certain example embodiments;

FIG. 4 is a flowchart illustrating a second example technique for dopinggraphene in accordance with certain example embodiments;

FIG. 5 is an example schematic view illustrating a third exampletechnique for doping graphene in accordance with certain exampleembodiments;

FIG. 6 is a graph plotting temperature vs. time involved in the dopingof graphene in accordance with certain example embodiments;

FIG. 7 is an example layer stack useful in the graphene release ordebonding techniques of certain example embodiments;

FIG. 8 is an example schematic view of a lamination apparatus that maybe used to dispose the graphene on the target glass substrate inaccordance with certain example embodiments;

FIG. 9 is a cross-sectional schematic view of a reactor suitable fordepositing high electronic grade (HEG) graphene in accordance with anexample embodiment;

FIG. 10 is an example process flow that illustrates certain of theexample catalytic CVD growth, lift-off, and transfer techniques ofcertain example embodiments;

FIG. 11 is an image of a sample graphene produced according to certainexample embodiments;

FIG. 12 is a cross-sectional schematic view of a solar photovoltaicdevice incorporating graphene-based layers according to certain exampleembodiments;

FIG. 13 is a cross-sectional schematic view of a touch screenincorporating graphene-based layers according to certain exampleembodiments; and

FIG. 14 is a flowchart illustrating an example technique for forming aconductive data/bus line in accordance with certain example embodiments;

FIG. 15 is a schematic view of a technique for forming a conductivedata/bus line in accordance with certain example embodiments;

FIG. 16 is an example precursor that is both a PAH and discotic;

FIG. 17 shows example PAH molecules with varying numbers, N, ofhexagonal carbons or sextets. Depicted in FIG. 17 are N=10, 17, and 18;

FIG. 18 illustrates the variation of LUMO-HOMO energy difference forcarbon molecules and PAH molecules with varying numbers, N, of hexagonalcarbons or sextets;

FIG. 19 shows the Raman Spectra and the G′ peak of the graphene filmsgrown in accordance with certain example embodiments;

FIGS. 20( a) and 20(b) show one possible route to HBC; and

FIG. 21 shows a molecule of HBC-PhCl2.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS OF THE INVENTION

Certain example embodiments of this invention relate to a scalabletechnique to hetero-epitaxially grow mono-crystalline graphite (n aslarge as about 15) and convert it to high electronic grade (HEG)graphene (n< about 3). Certain example embodiments also relate to theuse of HEG graphene in transparent (in terms of both visible andinfrared spectra), conductive ultra-thin graphene films, e.g., as analternative to the more ubiquitously employed metal oxides windowelectrodes for a variety of applications (including, for example,solid-state solar cells). The growth technique of certain exampleembodiments is based on a catalytically driven hetero-epitaxial CVDprocess which takes place a temperature that is low enough to beglass-friendly. For example, thermodynamic as well as kineticsprinciples allow HEG graphene films to be crystallized from the gasphase on a seed catalyst layer (e.g., at a temperature less than about600 degrees C.).

FIG. 1 is a high level flowchart illustrating the overall techniques ofcertain example embodiments. As shown in FIG. 1, the overall techniquesof certain example embodiments can be classified as belonging to one offour basic steps: graphene crystallization on a suitable back support(step S101), graphene release or debonding from the back support (stepS103), graphene transference to the target substrate or surface (stepS105), and incorporation of the target substrate or surface into aproduct (step S107). As explained in greater detail below, it will beappreciated that the product referred to in step S107 may be anintermediate product or a final product.

Example Graphene Crystallization Techniques

The graphene crystallization techniques of certain example embodimentsmay be thought of as involving “cracking” a hydrocarbon gas andre-assembling the carbon atoms into the familiar honeycomb structureover a large area (e.g., an area of about 1 meter, or larger), e.g.,leveraging the surface catalytic pathway. The graphene crystallizationtechniques of certain example embodiments take place at high temperatureand moderate pressures. Illustrative details of this example processwill be described in detail below.

The catalytic growth techniques of certain example embodiments aresomewhat related to the techniques that have been used to grow graphiteover a hetero-epitaxial area. A catalyst for graphene crystallization isdisposed on a suitable back support. The back support may be anysuitable material capable of withstanding high heats (e.g., temperaturesup to about 1000 degrees C.) such as, for example, certain ceramics orglass products, zirconium inclusive materials, aluminum nitridematerials, silicon wafers, etc. A thin film is disposed, directly orindirectly, on the back support, thereby ensuring that its surface issubstantially uncontaminated prior to the crystallization process. Theinventor of the instant invention has discovered that graphenecrystallization is facilitated when the catalyst layer has asubstantially single-orientation crystal structure (e.g., fewer creasesare formed). In this regard, small grains have been determined to beless advantageous, since their mosaic structure ultimately will betransferred to the graphene layer. In any event, the particularorientation of the crystal structure has been found to be largelyinsignificant for graphene crystallization, provided that the catalystlayer, at least in substantial part, has a single-orientation crystalstructure. Indeed, the comparative absence of (or low) grain boundariesin the catalyst has been found to result in the same or a similarorientation for the grown graphene, and has been found to provide forhigh electrical grade (HEG) graphene.

The catalyst layer itself may be disposed on the back support by anysuitable technique such as, for example, sputtering, combustion vapordeposition (CVD), flame pyrolysis, etc. The catalyst layer itself maycomprise any suitable metal or metal-inclusive material. For instance,the catalyst layer may comprise, for example, metals such as nickel,cobalt, iron, permalloy (e.g., nickel iron alloys, generally comprisingabout 20% iron and 80% nickel), alloys of nickel and chromium, copper,and combinations thereof. Of course, other metals may be used inconnection with certain example embodiments. The inventor has discoveredthat catalyst layers of or including nickel are particular advantageousfor graphene crystallization, and that alloys of nickel and chromium areyet more advantageous. Furthermore, the inventor has discovered that theamount of chromium in nickel-chromium layers (also sometimes callednichrome or NiCr layers) can be optimized so as to promote the formationof large crystals. In particular, 3-15% Cr in the NiCr layer ispreferable, 5-12% Cr in the NiCr layer is more preferable, and 7-10% ofCr in the NiCr layer is still more preferable. The presence of vanadiumin the metal thin film also has been found to be advantageous to promotelarge crystal growth. The catalyst layer may be relatively thin orthick. For example, the thin film may be 50-1000 nm thick, morepreferably 75-750 nm thick, and still more preferably 100-500 nm thick.A “large crystal growth” may in certain example instances includecrystals having a length along a major axis on the order of 10 s ofmicrons, and sometimes even larger.

Once the catalyst thin film is disposed on the back support, ahydrocarbon gas (e.g., C₂H gas, CH₄ gas, etc.) is introduced in achamber in which the back support with the catalyst thin film disposedthereon is located. The hydrocarbon gas may be introduced at a pressureranging from about 5-150 mTorr, more prerferably 10-100 mTorr. Ingeneral, the higher the pressure, the faster the growth of the graphene.The back support and/or the chamber as a whole is/are then heated todissolve or “crack open” the hydrocarbon gas. For example, the backsupport may be raised to a temperature in the range of 600-1200 degreesC., more preferably 700-1000 degrees C., and still more preferably800-900 degrees C. The heating may be accomplished by any suitabletechnique such as, for example, via a short wave infrared (IR) heater.The heating may take place in an environment comprising a gas such asargon, nitrogen, a mix of nitrogen and hydrogen, or other suitableenvironment. In other words, the heating of the hydrocarbon gas may takeplace in an environment comprising other gasses in certain exampleembodiments. In certain example embodiments, it may be desirable to usea pure hydrocarbon gas (for example, with C₂H₂), whereas it may bedesirable to use a mix of hydrocarbon gas an another inert or other gas(for example, CH₄ mixed with Ar).

The graphene will grow in this or another suitable environment. To stopthe growth and to help ensure that the graphene is grown on the surfaceof the catalyst (e.g., as opposed to being embedded within thecatalyst), certain example embodiments employ a quenching process. Thequenching may be performed using an inert gas such as, for example,argon, nitrogen, combinations thereof, etc. To promote graphene growthon the surface of the catalyst layer, the quenching should be performedfairly quickly. More particularly, it has been found that quenching toofast or too slow results in poor or no graphene growth on the surface ofthe catalyst layer. Generally, quenching so as to reduce the temperatureof the back support and/or substrate from about 900 degrees C. to 700degrees (or lower) over the course of several minutes has been found topromote good graphene growth, e.g., via chemisorption. In this regard,FIG. 2 is an example schematic view of the catalytic growth techniquesof certain example embodiments, illustrating the introduction of thehydrocarbon gas, the carbon dissolving, and the possible results ofquenching, in accordance with certain example embodiments.

The growth process of graphene imposes the strict film thicknessrelation t=n×SLG, where n involves some discrete number of steps.Identifying very rapidly if graphene has been produced and determiningthe value of n over the film area is roughly equivalent to measuringfilm quality and uniformity in one single measurement. Although graphenesheets can be seen by atomic force and scanning electron microscopy,these techniques are time consuming and can also lead to contaminationof the graphene. Therefore, certain example embodiments employ a phasecontrast technique that enhances the visibility of graphene on theintended catalyst surfaces. This may be done with a view to mapping anyvariation in n-value over the deposition surface on the metalliccatalyst film. The technique relies on the fact that the contrast ofgraphene may be enhanced substantially by spin coating a material ontoit. For example, a widely used UV curable resist (e.g., PMMA) may bespin coated, screen printed, gravure coated, or otherwise disposed onthe graphene/metal/back support, e.g., at a thickness sufficient to makethe film visible and continuous (e.g., around 1 micron thick). Asexplained in greater detail below, the inclusion of a polymer resistalso may facilitate the lift-off process of the graphene prior to itstransfer to the final surface. That is, in addition to providing anindication as to when graphene formation is complete, the polymer resistalso may provide a support for the highly elastic graphene when themetal layer is released or otherwise debonded from the back support asexplained in detail below.

In the event that a layer is grown too thick (either intentionally orunintentionally), the layer can be etched down, for example, usinghydrogen atoms (H*). This technique may be advantageous in a number ofexample situations. For instance, where growth occurs too quickly,unexpectedly, unevenly, etc., H* can be used to correct such problems.As another example, to ensure that enough graphene is grown, graphitemay be created, graphane may be deposited, and the graphane may beselectively etched back to the desired n-level HEG graphene, e.g., usingH*. As still another example, H* can be used to selectively etch awaygraphene, e.g., to create conductive areas and non-conductive areas.This may be accomplished by applying an appropriate mask, performing theetching, and then removing the mask, for example.

Theoretical studies of graphene has shown that the mobility of carrierscan be higher than 200,000 cm²/(V·s). Experimental measurements of gasphase treated hetero-epitaxial grown graphene show resistivity as low as3×10⁻⁶ Ω-cm, which is better than that of silver thin films. The sheetresistance for such graphene layers has been found to be about 150ohms/square. One factor that may vary is the number of layers ofgraphene that are needed to give the lowest resistivity and sheetresistance, and it will be appreciated that the desired thickness of thegraphene may vary depending on the target application. In general,graphene suitable for most applications may be n=1-15 graphene, morepreferably n=1-10 graphene, still more preferably n=1-5 graphene, andsometimes n=2-3 graphene. An n=1 graphene layer has been found to resultin a transmission drop of about 2.3-2.6%. This reduction in transmissionhas been found to be generally linear across substantially all spectra,e.g., ranging from ultraviolet (UV), through visible, and through IR.Furthermore, the loss in transmission has been found to be substantiallylinear with each successive incrementation of n.

Example Doping Techniques

Although a sheet resistance of 150 ohms/square may be suitable forcertain example applications, it will be appreciated that a furtherreduction in sheet resistance may be desirable for different exampleapplications. For example, it will be appreciated that a sheetresistance of 10-20 ohms/square may be desirable for certain exampleapplications. The inventor of the instant invention has determined thatsheet resistance can be lowered via the doping of the graphene.

In this regard, being only one atomic layer thick, graphene exhibitsballistic transport on a submicron scale and can be doped heavily—eitherby gate voltages or molecular adsorbates or intercalates in the casewhere n≧2—without significant loss of mobility. It has been determinedby the inventor of the instant invention that that in graphene, asidefrom the donor/acceptor distinction, there are in general two differentclasses of dopants, namely, paramagnetic and nonmagnetic. In contrast toordinary semiconductors, the latter type of impurities act generally asrather weak dopants, whereas the paramagnetic impurities cause strongdoping: Because of the linearly vanishing, electron-hole symmetricdensity of states (DOS) near the Dirac point of graphene, localizedimpurity states without spin polarization are pinned to the center ofthe pseudo-gap. Thus, impurity states in graphene distinguish stronglyfrom their counterparts in usual semiconductors, where the DOS in thevalence and conduction bands are very different and impurity levels liegenerally far away from the middle of the gap. Although one might notexpect a strong doping effect which requires existence of well-defineddonor (or acceptor) levels several tenths of electron volt away from theFermi level, if the impurity has a local magnetic moment, its energylevels split more or less symmetrically by the Hund exchange, of theorder of 1 eV, which provides a favorable situation for a strong dopingimpurity effects on the electronic structure of two-dimensional systemswith Dirac-like spectrum such as those present in graphene. This line ofreasoning may be used to guide the choice of molecules that form bothparamagnetic single molecules and diamagnetic dimers systems to dopegraphene and enhance its conductivity from 10³ S/cm to 10⁵ S/cm, andsometimes even to 10⁶ S/cm.

Example dopants suitable for use in connection with certain exampleembodiments include nitrogen, boron, phosphorous, fluorides, lithium,potassium, ammonium, etc. Sulfur-based dopants (e.g., sulfur dioxide,sulfuric acid, hydrogen peroxide, etc.) also may be used in connectionwith certain example embodiments. For example, sulfites present in glasssubstrates may be caused to seep out of the glass and thus dope thegraphene-based layer. Several example graphene doping techniques are setforth in greater detail below.

FIG. 3 is a flowchart illustrating a first example technique for dopinggraphene in accordance with certain example embodiments. The FIG. 3example technique essentially involves ion beam implanting the dopingmaterial in the graphene. In this example technique, graphene is grownon a metal catalyst (step S301), e.g., as described above. The catalystwith the graphene formed thereon is exposed to a gas comprising amaterial to be used as the dopant (also sometimes referred to as adopant gas) (step S303). A plasma is then excited within a chambercontaining the catalyst with the graphene formed thereon and the dopantgas (S305). An ion beam is then used to implant the dopant into thegraphene (S307). Example ion beam techniques suitable for this sort ofdoping are disclosed in, for example, U.S. Pat. Nos. 6,602,371;6,808,606; and Re. 38,358, and U.S. Publication No. 2008/0199702, eachof which is hereby incorporated herein by reference. The ion beam powermay be about 10-200 ev, more preferably 20-50 ev, still more preferably20-40 ev.

FIG. 4 is a flowchart illustrating a second example technique for dopinggraphene in accordance with certain example embodiments. The FIG. 4example technique essentially involves pre-implanting solid statedopants in the target receiving substrate, and then causing those solidstate dopants to migrate into the graphene when the graphene is appliedto the receiving substrate. In this example technique, graphene is grownon a metal catalyst (step S401), e.g., as described above. The receivingsubstrate is pre-fabricated so as to include solid-state dopants therein(step S403). For example, the solid-state dopants may be included viathe melting in the formulation in the glass. About 1-10 atomic %, morepreferably 1-5 atomic %, and still more preferably 2-3 atomic % dopantmay be included in the glass melt. The graphene is applied to thereceiving substrate, e.g., using one of the example techniques describedin detail below (step S405). Then, the solid-state dopants in thereceiving substrate are caused to migrate into the graphene. The heatused in the deposition of the graphene will cause the dopants to migratetowards the graphene layer being formed. Similarly, additionally dopedfilms can be included on the glass and the dopants therein can be causedto migrate through these layers by thermal diffusion, for example,creating a doped graphene (n>=2) layer.

An ion beam also can be used to implant the dopants directly in theglass in certain example embodiments. The ion beam power may be about10-1000 ev, more preferably 20-500 ev, still more preferably 20-100 ev.When an intermediate layer is doped and used to provide impurities forthe graphene, the ion beam may operate at about 10-200 ev, morepreferably 20-50 ev, still more preferably 20-40 ev.

FIG. 5 is an example schematic view illustrating a third exampletechnique for doping graphene in accordance with certain exampleembodiments. The FIG. 5 example techniques essentially involvespre-implanting solid state dopants 507 in the metal catalyst layer 503,and then causing those solid state dopants 507 to migrate through thecatalyst layer 503 as the graphene is being formed, thereby creating adoped graphene 509 on the surface of the catalyst layer 503. Moreparticularly, in this example technique, the catalyst layer 503 isdisposed on the back support 505. The catalyst layer 503 includessolid-state dopants 507 therein. In other words, the catalyst hassolid-state dopant atoms inside its bulk (e.g., from about 1-10%, morepreferably about 1-5%, and most preferably about 1-3%). Hydrocarbon gas501 is introduced proximate to the catalyst layer 503 formed, at a hightemperature. The solid-state dopants 507 in the catalyst layer 503 arecaused to migrate towards the outer surface thereof, e.g., by this hightemperature, as the graphene crystallization takes place. The rate atwhich the dopants arrive at the surface has been found to be a functionof the catalyst thickness and temperature. The crystallization isstopped via quenching and, ultimately, a doped graphene 509 is formed onthe surface of the catalyst layer 503′. Following the formation of thedoped graphene 509, the catalyst layer 503′ now has fewer (or no)solid-state dopants 507 located therein. One advantage of this exampletechnique relates to the potential to control the ultrathin film growthby judiciously varying the metal surface temperature, partial pressure,and residence time of the deposition gas species, as well as thereactive radicals used in quenching rate process.

It will be appreciated that these example doping techniques may be usedalone and/or in various combinations and sub-combinations with oneanother and/or further techniques. It also will be appreciated thatcertain example embodiments may include a single dopant material ormultiple dopant materials, e.g., by using a particular example techniqueonce, a particular technique repeatedly, or through a combination ofmultiple techniques one or more times each. For example, p-type andn-type dopants are possible in certain example embodiments.

FIG. 6 is a graph plotting temperature vs. time involved in the dopingof graphene in accordance with certain example embodiments. As indicatedabove, the cooling may be accomplished using, for example, an inert gas.In general, and also as indicated above, the high temperature may beabout 900 degrees C. in certain example embodiments, and the lowtemperature may be about 700 degrees C., and the cooling may take placeover several minutes. The same heating/cooling profile as that shown inFIG. 6 may be used regardless of whether the graphene is doped.

Example Graphene Release/Debonding and Transfer Techniques

Once graphene has been hetero-epitaxially grown, it may be released ordebonded from the metal catalyst and/or the back support, e.g., prior tobeing placed on substrate to be incorporated into the intermediate orfinal product. Various procedures may be implemented for liftingepitaxial films from their growth substrates in accordance with certainexample embodiments. FIG. 7 is an example layer stack useful in thegraphene release or debonding techniques of certain example embodiments.Referred to FIG. 7, in certain example embodiments, an optional releaselayer 701 may be provided between the back support 505 and the catalystlayer 503. This release layer 701 may be of or include, for example,zinc oxide (e.g., ZnO or other suitable stoichiometry). Post-graphenedeposition, the graphene 509/metal catalyst layer 503/release layer 701stack coated substrate 505 may receive a thick overcoat (e.g., severalmicrons thick) layer of polymer 703, e.g., applied via spin coating,dispensed by a meniscus flow, etc., which may be cured. As alluded toabove, this polymer layer 703 may act as a backbone or support for thegraphene 509 during lift-off and/or debonding, keeping the extremelyflexible graphene film continuous, while also reducing the likelihood ofthe graphene film curling up, creasing, or otherwise deforming.

Also as alluded to above, PMMA may be used as the polymer that allowsthe graphene to become visible by phase contrast and for support priorto and/or during lift-off. However, a broad range of polymers whosemechanical and chemical properties can be matched to those of graphenemay be used during the support phase, as well as the release transferphase in connection with certain example embodiments. The work forlift-off may be performed in parallel with the main epitaxial growthbranch, e.g., by experimenting with graphene films that can bechemically exfoliated from graphite.

The release layer may be chemically induced to de-bond thegraphene/metal from mother substrate once the polymer layer disposedthereon. For example, in the case of a zinc oxide release layer, washingin vinegar may trigger the release of the graphene. The use of a zincoxide release layer also is advantageous, inasmuch as the inventor ofthe instant invention has discovered that the metal catalyst layer alsois removed from the graphene with the release layer. It is believed thatthis is a result of the texturing caused by the zinc oxide release layertogether with its inter-linkages formed with the grains in the catalystlayer. It will be appreciated that this reduces (and sometimes eveneliminates) the need to later remove the catalyst layer.

Certain lift-off/debonding and transfer techniques essentially regardthe original substrate as a reusable epitaxial growth substrate. Assuch, a selective etching to undercut and dissolve away the metalliccatalyst thin film away from the epitaxially grown (with polymer on top)graphene may be desirable in such example embodiments. Thus, thecatalyst layer may be etched off, regardless of whether a release layeris used, in certain example embodiments. Suitable etchants include, forexample, acids such as hydrochloric acid, phosphoric acid, etc.

The final recipient glass substrate surface may be prepared so as toreceive the graphene layer. For example, a Langmuir Blodgett film (e.g.,from a Langmuir-Blodgett acid) may be applied to the glass substrate.The final recipient substrate alternatively or additionally may becoated with a smooth graphenophillic layer such as, for example, asilicone-based polymer, etc., making the latter receptive to thegraphene. This may help to ensure electrostatic bonding, thuspreferentially allowing the transfer of the graphene during transfer.The target substrate may additionally or alternatively be exposed to UVradiation, e.g., to increase the surface energy of the target substrateand thus make it more receptive to the graphene.

The graphene may be applied to the substrate via blanket stamping and/orrolling in certain example embodiments. Such processes allow thegraphene previously grown and chemisorbed onto the metal carrier to betransferred onto the recipient glass by contact pressure. As oneexample, the graphene may be applied to the substrate via one or morelamination rollers, e.g., as shown in FIG. 8. In this regard, FIG. 8shows upper and lower rollers 803 a and 803 b, which will apply pressureand cause the graphene 509 and polymer layer 703 to be laminated to thetarget substrate 801. As noted above, the target substrate 801 have asilicon-inclusive or other graphenophillic layer disposed thereon tofacilitate the lamination. It will be appreciated that the polymer layer703 will be applied as the outermost layer and that the graphene 509will be closer to (or even directly on) the target substrate 801. Incertain example embodiments, one or more layers may be provided on thesubstrate prior to the application of the graphene.

Once the graphene is disposed on the target substrate, the polymer layermay be removed. In certain example embodiments, the polymer may bedissolved using an appropriate solvent. When a photosensitive materialsuch as PMMA is used, it may be removed via UV light exposure. Ofcourse, other removal techniques also are possible.

It will be appreciated that the catalyst thin film may be etched offafter the graphene has been applied to the target substrate in certainexample embodiments, e.g., using one of the example etchants describedabove. The choice of etchant also may be based on the presence orabsence of any layers underlying the graphene.

Certain example embodiments more directly electrochemically anodize themetal catalyst thin film below the graphene. In such exampleembodiments, the graphene itself may act as the cathode, as the metalbelow is anodized into a transparent oxide while still being bonded tothe original substrate. Such example embodiments may be used to bypassthe use of the polymer overcoat by essentially performing the lift-offand transfer processes in one step. However, anodization byelectrochemical means may affect the electronic properties of thegraphene and thus may need to be compensated for. In certain exampleembodiments, the catalyst layer below the graphene may be oxidized inother ways to make it transparent. For example, a conductive oxide maybe used to “link” the graphene-based layer to a substrate,semiconductor, or other layer. In this regard, cobalt, chrome cobalt,nickel chrome cobalt, and/or the like may be oxidized. In certainexample embodiments, this may also reduce the need for graphenelift-off, making the transfer, manipulation, and other handling ofgraphene easier.

The graphene also may be picked up using an adhesive or tape-likematerial in certain example embodiments. The adhesive may be positionedon the target substrate. The graphene may be transferred to the targetsubstrate, e.g., following the application of pressure, by more stronglyadhering to the substrate than to the tape, etc.

Example Reactor Design

Showerhead reactors typically employ a perforated or porous planarsurface to dispense reactant gasses more-or-less uniformly over a secondparallel planar heated surface. Such a configuration may be used to growgraphene using the example hetero-epitaxial techniques described herein.Showerhead reactors also are advantageous for the processing of largesquare ultra-smooth glass or ceramic substrate. A basic schematic of ashowerhead reactor is FIG. 9, with the plenum design being enlarged. Inother words, FIG. 9 is a cross-sectional schematic view of a reactorsuitable for depositing high electronic grade (HEG) graphene inaccordance with an example embodiment. The reactor includes a bodyportion 901 with several inlets and outlets. More particularly, a gasinlet 903 is provided at the top and in the approximate horizontalcenter of the body portion 901 of the reactor. The gas inlet 903 mayreceive gas from one or more sources and thus may provide various gassesincluding, for example, the hydrocarbon gas, the gas(ses) used to formthe environment during hetero-epitaxial growth, the quenching gas(ses),etc. The flow and flux of the gas will be described in greater detailbelow, e.g., with reference to the plenum design of the showerhead 907.A plurality of exhaust ports 905 may be provided at the bottom of thebody portion 901 of the reactor. In the FIG. 9 example embodiment, twoexhaust ports 905 are provided proximate to the extremes of the bodyportion 901 of the reactor, e.g., so as to draw out gas provided by thegas inlet 903 that generally will flow through substantially theentirety of the body portion 901. It will be appreciated that more orfewer exhaust ports 905 may be provided in certain example embodiments(e.g., further exhaust ports 905 may be provided in the approximatehorizontal center of the body portion 901 of the reactor, at the top orsides of the body portion 901 of the reactor, etc.).

The back support substrate 909 may be cleaned and have the catalyst thinfilm disposed thereon (e.g., by physical vapor deposition or PVD,sputtering, CVD, flame pyrolysis, or the like) prior to entry into thereactor by a load-lock mechanism in certain example embodiments. Interms of susceptor design, the surface of the back support substrate 909may be rapidly heated (e.g., using an RTA heater, a shortwave IR heater,or other suitable heater that is capable of inductively heating thesubstrate and/or layers thereon without necessarily also heating theentire chamber) to a controllable temperature level and uniformity thatallows (i) the metal film to crystallize and activate, and (ii) thepreferential deposition of graphene of substantially uniform andcontrollable thickness from a gas phase precursor on its surface. Theheater may be controllable so as to account for the parameter depositionrate/(temperature*thickness) of catalyst ratio. The back supportsubstrate 909 may move through the reactor in the direction R or may sitstationary under the showerhead 907. The showerhead 907 may be cooled,e.g., using a cooling fluid or gas introduced by one or more coolantinlets/outlets 913. In brief, and as shown in the FIG. 9 enlargement,the plenum design may include a plurality of apertures in the bottom ofthe showerhead 907, with each such aperture being only a few millimeterswide.

Changing the ceiling gap Hc, or the height between the bottom surface ofthe showerhead 907 and the top surface upon which the back supportsubstrate 909 moves, may have several effects. For example, the chambervolume and thus the surface-to-volume ratio may be modified, therebyaffecting the gas residence time, consumption time, and radialvelocities. Changes in the residence time have been found to stronglyinfluence the extent of gas phase reactions. A showerhead configurationoperated as shown in FIG. 9 (with a hot surface below a cooled surface)has the potential for Benard variety natural convection if operated athigh pressures (e.g., in the hundreds of Torr), and such a tendency isstrongly influenced by the height through the Rayleigh number (adimensionless number associated with buoyancy driven flow, also known asfree convection or natural convection; when it exceeds a critical valuefor a fluid, heat transfer is primarily in the form of convection).Accordingly, the ceiling gap Hc may be varied through simple hardwarechanges, by providing adjustable mounting of the substrate electrode,etc., so as to affect the hetero-epitaxial growth of the graphene.

The FIG. 9 example embodiment is not necessarily intended to operate aplasma within the reactor. This is because the crystalline film growthmechanism is by hetero-epitaxy by surface sorption (generally occurringonly on the catalyst). Growth from the plasma phase has been found togive rise to mostly amorphous films and also has been found to allow theformation of macro-particle or dust formation that can greatly reducefilm quality and result in pinholes that would be detrimental for aone-to-ten atomic layer film. Instead, certain example embodiments mayinvolve making graphite (e.g., monocrystalline graphite), etching it tographane (e.g., of a certain n-value), and turning the graphane intographene (e.g., into HEG graphene). Of course, an in-situ end-pointtechnique may be implemented as a feed-back parameter.

In certain example embodiments, an ion beam source may be locatedin-line but external to the reactor of FIG. 9, e.g., to perform dopingin accordance with the example techniques described above. However, incertain example embodiments, an ion beam source may be located withinthe body portion of a reactor.

Example Process Flow

FIG. 10 is an example process flow that illustrates certain of theexample catalytic CVD growth, lift-off, and transfer techniques ofcertain example embodiments. The example process shown in FIG. 10 beginsas the back support glass is inspected, e.g., using a conventional glassinspection method (step S1002) and washed (step S1004). The back supportglass may then be cleaned using ion beam cleaning, plasma ashing, or thelike (step S1006). The catalyst (e.g., a metal catalyst) is disposed onthe back support, e.g., using PVD (step S1008). It is noted that thecleaning process of step S1006 may be accomplished within the graphenecoater/reactor in certain example embodiments of this invention. Inother words, the back support glass with or without the metal catalystthin film formed thereon may be loaded into the graphene coater/reactorprior to step S1006 in certain example embodiments, e.g., depending onwhether the metal catalyst layer is deposited within or prior to thecoater/reactor. The catalytic deposition of an n-layer graphene may thentake place (step S1010). The graphene may be etched down by introducinghydrogen atoms (H*) in certain example embodiments, and the grapheneoptionally may be doped, e.g., depending on the target application (stepS1012). The end of the graphene formation is detected, e.g., bydetermining whether enough graphene has been deposited and/or whetherthe H* etching has been sufficient (step S1014). To stop the grapheneformation, a rapid quenching process is used, and the back support glasswith the graphene formed therein exits the reactor/coater (step S1016).Visual inspection optionally may be performed at this point.

Following graphene formation, a polymer useful in the transference ofthe graphene may be disposed on the graphene, e.g., by spin, blade, orother coating technique (step S1018). This product optionally may beinspected, e.g., to determine whether the requisite color change takesplace. If it has, the polymer may be cured (e.g., using heat, UVradiation, etc.) (step S1020), and then inspected again. The metalcatalyst may be under-etched or otherwise released (step S1022), e.g.,to prepare the graphene for lift-off (step S1024).

Once lift-off has been achieved, the polymer and the graphene optionallymay be inspected and then washed, e.g., to remove any remainingunder-etchants and/or non-cured polymer (step S1026). Another optionalinspection process may be performed at this point. A surfactant may beapplied (step S1028), pins are placed at least into the polymer (stepS1030), and the membrane is flipped (step S1032), e.g., with the aid ofthese pins. The lift-off process is now complete, and the graphene isnow ready to be transferred to the recipient substrate.

The recipient substrate is prepared (step S1034), e.g., in a clean room.The surface of the recipient substrate may be functionalized, e.g., byexposing it to a UV light to increase its surface energy, to applygraphenophillic coatings thereto, etc. (step S1036). Thegraphene/polymer membrane may then be transferred onto the hostsubstrate (step S1038).

Once the transfer is complete, the receiving substrate with the grapheneand polymer attached thereto may be fed into a module to remove thepolymer (step S1040). This may be done by exposing the polymer to UVlight, heat, chemicals, etc. The substrate with the graphene and atleast partially dissolved polymer may then be washed (step S1042), withany excess water or other materials evaporated and dried off (stepS1044). This polymer removal process may be repeated, as necessary.

Following the removal of the polymer, the sheet resistance of thegraphene on the substrate may be measured (step S1046), e.g., using astandard four-point probe. Optical transmission (e.g., Tvis, etc.) alsomay be measured (step S1048). Assuming that the intermediate or finalproducts meet quality standards, they may be packaged (step S1050).

Using these techniques, sample films were prepared. The sample filmsexhibited high conductivity of 15500 S/cm and transparency of more than80% over the 500-3000 nm wavelength. Furthermore, the films showed goodchemical and thermal stability. FIG. 11 is an image of a sample grapheneproduced according to certain example embodiments. The FIG. 11 imagehighlights the lift-off of the hetero-epitaxially grown graphene from apermalloy thin film.

Example Deposition of Graphene Directly or Indirectly on Glass, WithoutLiftoff

As explained in detail above, high-quality graphene may be epitaxiallygrown using catalytic CVD techniques. However, the techniques describedabove generally involved a lift-off process of the graphene sheet fromthe thin metallic film catalyst. These techniques have issues that maysometimes affect the electrical conductivity of the resulting filmsincluding, for example, damage of the graphene films during etching ofthe metal, creasing of the graphene during transfer, etc. One factorthat may sometimes affect the density of creasing and strain in thesheet relates to the potential mismatch in thermal expansion between themetal catalyst and the grown graphene, especially during the rapidcooling often used when forming graphene. The potential for creatingdefects to the graphene film may be a barrier to developing alarge-scale process for making a high-quality optoelectronic-grade ofthe material. Therefore, it will be appreciated that there is a need inthe art to grow graphene on any appropriate given substrate (such as,for example, metal, semiconductor, and/or glass substrates).

Certain example embodiments provide alternate techniques that bypass theneed for a liftoff process. More particularly, certain exampleembodiments of this invention involve the hetero-epitaxial growth ofgraphene from a supramolecular species (that inherently have multiplearomatic rings). In this regard, graphene may be grown in certainexample embodiments from supramolecules that self-arrange themselves ona substrate, and the system may then be very gradually heated in anappropriate atmosphere (e.g., an inert atmosphere, under the presence ofan inert gas, in an inert/hydrocarbon mixture including, for example,acetylene, etc.) to produce high-quality grade graphene. This techniquemay involve a slow ramp up of temperature, e.g., from room temperature,to just above 400 degrees C., and then more rapidly to as high as about800 degrees C. This example process advantageously may reduce (andsometimes even completely eliminate) the need for rapid thermalquenching and lift-off and transfer. In certain example embodiments, theslow temperature ramp may be performed using, for example, a highlycontrollable long-wave IR lamp and then activating a short IR lampthereafter to increase the temperature to above 600 degrees C. In anyevent, such example embodiments rely on the inventor's examination ofcertain precursors that belong to a class of large molecules that areboth polycyclic aromatic hydrocarbons (PAHs) and discotic. One suchexample precursor that is both a PAH and discotic is shown in FIG. 16.It has been discovered that C96 and C34 (e.g., C96-C12 and C34-C12) PAHmolecules, and derivatives thereof, may be used in connection withcertain example embodiments of this invention.

Polycyclic aromatic hydrocarbons (also sometimes called polynuclearhydrocarbons) have two or more single or fused aromatic rings if a pairof carbon atoms is shared between rings in their molecules. The term“PAH” generally refers to compounds comprising carbon and hydrogenatoms, while the wider term “polycyclic aromatic compounds” includes thealkyl-substituted derivatives and functional derivatives, such as nitro-and hydroxy-PAH as well as the heterocyclic analogues, which contain oneor more hetero atoms in the aromatic structure. PAHs exist in variouscombinations that manifest various functions such as light sensitivity,heat resistance, conductivity, corrosion resistance and physiologicalaction. The simplest examples are naphthalene having two benzene ringsside by side, and biphenyl having two bond-connected benzene rings. PAHsare not found in synthetic products and are non-essential for the growthof living cells. The general characteristics of PAH include highmelting- and boiling-points (they are solid), low vapor pressure, andvery low water solubility, decreasing with increasing molecular weightwhereas resistance to oxidation, reduction, and vaporization increases.Vapor pressure tends to decrease with increasing molecular weight. PAHsare highly lipophilic and readily soluble in organic solvents. The lowermolecular weight PAHs of 2 or 3 ring groups such as naphthalenes,fluorenes, phenanthrenes, and anthracenes have toxicity that tends todecrease with increasing molecular weight. PAHs are not synthesizedchemically for industrial purposes but are isolated from concentratedcoal-tar products (or from pyrolysis of coal hydrocarbons) followed bysubsequent purification through repeated distillation andcrystallization.

As is known, the term “discotic” refers to the columnar stackability offlat, disc-like molecules.

Example approaches that bypass the need for a liftoff process aredescribed in greater detail below, and it will be appreciated that theyare still bottom-up approaches. However, such example approaches involvemolecules that are more complex than acetylene. The core of thesemolecules may in certain example instances be regarded as molecularsub-units of graphene. These molecules self-assemble into 1-dimensionalcolumnar supramolecular structures (or substantially 1-dimensional,substantially columnar supramolecular structures) by virtue of the largeπ-π interactions between the core aromatic regions. Using appropriatesolvents to first dissolve the PAHs, for example, simpleLangmuir-Blodgett techniques may be used to deposit PAHs onto glass. Asis known, Langmuir-Blodgett deposition generally involves the depositionof a material from the surface of a liquid onto a solid by immersing thesolid into the liquid. In general, a monolayer is adsorbed homogenouslywith each immersion or emersion step. In any event, the organization ofthese molecules into highly ordered layers akin to graphene is thenfacilitated. Upon a gradual heating process in vacuum and under inert(Ar, He, etc.) and/or other gasses, graphene may be formed, directly orindirectly, on the glass substrate. That is, it will be appreciated thatgraphene may be formed on glass substrates without the need for aliftoff process in certain example embodiments.

For soda lime glass, an underlayer comprising silicon oxide (e.g., SiO₂or other suitable stoichiometry) may be deposited on the glass by atechnique such as, for example, MSVD. Deposition also was possible on Siwafers, after a clean, about 50 nm thick silicon oxide underlayer hadbeen thermally grown thereon. In both such cases, a monolayer of PAH wascreated by photochemical attachment of the PAH to an immobilizedself-assembled monolayer (SAM) of a silane-benzophenone derivative. Thesubstrate was then heated up to 750 degrees C. The final productincluded a film of graphene comprising a mostly bi-layer showing theexpected Raman fingerprint (described in greater detail below), as wellas a sheet resistance in the range of 10-200 ohms/square depending, forexample, on the substrate, the type of PAH used, as well as the type ofinert gas and temperature profiles. Optical Tvis transmission of thefilm on glass ranged from 82-87.1%. The sheet resistance of the filmsformed ranged from 50 ohms/sq. to 120 ohms/sq.

It will be appreciated that different oxidized underlayers may be usedin connection with different example embodiments of this invention. Forexample, underlayers of or comprising silicon oxide, zinc oxide, and/ortransition metal oxides, may be used in connection with certain exampleembodiments.

The techniques of certain example embodiments are advantageous in thatno catalyst, no rapid heating and quenching, and/or no acetylene gas maybe required. The supramolecules instead may advantageously self-arrangeinto a structure that, when heated to about 750 degrees C., formsgraphene. It will be appreciated that the temperature at which the onsetof graphene growth takes place is not necessarily 750 degrees C. Indeed,simulations show that, provided monolayers of PAHs are present, grapheneformation may begin at temperatures as low as about 450 degrees C. underinert gas atmosphere. Subtle knowledge of how to prepare the surfacewith a SAM template so as to anchor the PAH molecules to the glasssurface so that the c-axis is perpendicular or substantiallyperpendicular (meaning they are flat or substantially flat) to the glassis advantageous when seeking to achieve this result. A detaileddescription is provided below. In brief, it will be appreciated thatequipment to dispense both the SAM template and PAH molecules alreadyexists. In certain example embodiments, the heating may be performed invacuum or under inert atmosphere, and such techniques advantageouslylend themselves to already-existing manufacturing lines, provided theprecursor can be made with a sufficiently high purity. Example equipmentand example process conditions that may be used in connection withcertain example embodiments are provided below.

In certain example embodiments, silane-benzophenone may be used as theanchor template for the PAH. This broad class of PAH molecules was firstsynthesized by S. Chandrasekhar at the Raman Institute in 1977 (see S.Chandrasekhar, Liquid Crystals, Cambridge University Press (1992), theentire contents of which are hereby incorporated herein by reference))and has been studied by researchers at the College de France (see, e.g.,F. Rondelez, D. Koppel, B. K. Sadashiva, Journale de Physique 43, 9(1982) and F. Rondelez et al., Journal de Physique 48, 1225-1234 (1987),each of which is hereby incorporated herein by reference). FIG. 17 showsexample PAH molecules with varying numbers, N, of hexagonal carbons orsextets. Depicted in FIG. 17 are N=10, 17, and 18. It will beappreciated from FIG. 17 that alkyl groups R are present at the edges ofthese molecules. The R groups act as anchors to the substrate so as toorient the PAH molecules in place perpendicular or substantiallyperpendicular to the c-axis.

The aromatic molecule perylene (C₂₀H₁₂) is planar in shape, with arather large intrinsic charge-carrier mobility at low temperatures. Ithas been determined that perylene may be grown in both a monolayer aswell as in a multilayer regime on a variety of metallic, semiconducting,and insulating substrates. The substrates were cleaned using an ion beamand in most cases were coated with highly textured thin films of variousmetals, semiconductors as well as insulators. For instance, the materialwas grown on both (100) and (111) Si as well as highly textured Cu(111). One surprising and unexpected result of these studies was thatthe planar organic perylene molecules were found to grow with theπ-plane oriented almost or completely parallel to the substrate, notonly in the monolayer but also in the multilayer regime. This situationalso is surprising and unexpected in that the growth was found to be ofepitaxial type and preceded via a layer-by-layer growth mode. ForCu(110) substrates, for instance, by using HREELS, LEED, and STM, ahighly-ordered monolayer was observed, with a subsequent transition intoa still highly ordered structure for the multilayer. This early workhelped spur investigation regarding the possibility of growing grapheneusing HBC and HBC-derivatives via a chemical route.

For example, when considering the growth of polyacenes such as pentaceneand perylene on solid substrates, it would be desirable to seek out thelargest molecular segment of a graphite plane that can be deposited flatdown using sublimation. Hexaperi-hexabenzocoronene (C₄₂H₁₈), or HBC isone candidate. It is one of the largest molecules to be suited fororganic molecular-beam epitaxy growth of aromatic molecules on solidsubstrates for applications in organic electronics. In previous work, ithas been shown that deposition onto the freshly cleaved pyrolyticgraphite (0001) substrates and on molybdenum disulfide (MoS₂) surfacescan be used to form highly ordered films that grow in a layer-by-layerfashion. This growth-mode, which has been observed to continue up tothicknesses of at least 10 nm, is exceptional as it leads to theformation of highly ordered multilayers with a structure different fromthat seen in bulk HBC.

For larger thicknesses, a Stransky-Krastanov molecular growth mode hasbeen reported, where a transition to the bulk crystal structure isobserved. Interestingly, deposition onto polycrystalline Au-substratesor oxidized Si(100)-surfaces did not lead to the formation of highlyoriented layers of HBC; instead, the results indicated a highlydisordered arrangement of the HBC. This apparent difference in thegrowth mode on graphite and MoS₂ on the one hand and on metal surfaceson the other hand is removed when deposition is carried out on clean,well-defined substrates prepared under UHV conditions. Generally, onclean metals the formation of highly ordered monolayers with the HBCmolecular planes orientated parallel to the substrate is seen [Au(100)Au(111)]. Further deposition was shown to result in the growth of highlyoriented HBC-adlayers up to a thickness of 2 nm on Au(111) and onCu(111) substrates, with the ring-planes aligned parallel to thesubstrate. For layers exceeding a thickness of approximately 2 nm, aloss of orientation was reported. However, recognizing that HBC may bechemically modified so that it becomes soluble, it was possible todeposit thin films via spin-coating or a Langmuir-Blodgett technique(described in greater detail below). If alkyl chains are attached to theHBC, in several cases liquid-crystalline behavior has been observedleading to the formation of columnar structures the so called discoticphase. An alignment of these columns can be obtained by varying theparticular preparation conditions including, for example, the pH valueof the water sub-phase.

Achieving liquid crystalline hexabenzocoronenes based materials may incertain example embodiments enable by improvements of the synthesisleading to hexabenzocoronene derivatives with six-fold alkysubstitution, design and development of molecular materials withimproved properties such as solubility and “processability,” andincorporation of the obtained molecules in optoelectronic devices suchas organic solar cells.

By a new synthetic protocol, it was possible to undertake aryl-aryl andaryl-alkyl couplings very late in the reaction sequence leading to alarge variety of substituted HBC derivatives. The introduction of aphenyl spacer between the HBC core and the pending alkyl chains as inHBC-PhCl2 had a number of beneficial effects such as, for example,increased solubility and liquid crystallinity at room temperature, whichhelped ensure the formation of highly ordered films on a variety ofsubstrates necessary for the implementation in organic moleculardevices.

According to E. Clar (see Aromatic Sextet, 1972), the more sextetspresent in the PAH, the greater the thermal stability of the material.According to Clar, they are “super-benzenoid.” Triphenylene derived fromcoal tar is extremely stable. One possible route to HBC is shown, forexample, in FIGS. 20( a) and 20(b), and a molecule of HBC-PhCl2 is shownin FIG. 21.

A brief description of certain example techniques for creating anexample monolayer on a substrate will now be provided. A C96 (and/or HBCor hexabenzocoronene) monolayer was created by covalently binding achlorosilane benzophenone derivative to a semiconductor substrate as theanchor template. The photochemically reactive benzophenone functionalgroup was used to covalently bind the desired PAH molecule to thesurface. The presence of an alkyl chain in the PAH molecule helps it tobecome covalently attached with benzophenone upon irradiation. As such,certain example embodiments may include materials containing alkylchains as the source for covalent attachment to the benzophenone. Abenzophenone derivative was chosen as the linker between the glasssurface and C96 because of the chemical stability, ease of activationwith light at 340-360 nm, and its preferential reactivity, sometimeswith high specificity, with otherwise unreactive C-H bonds. Even in thepresence of solvent (e.g., water) and nucleophiles, the reactivitytowards unreactive C—H bonds was sustained. A technique involvingphotochemical attachment to the surface also allowed any non-bonded C96or HBC molecules to be washed away very easily. Upon irradiation atabout 350 nm, the benzophenone undergoes a transition where one electronfrom a non-bonding sp² like n-orbital on the oxygen moves to anantibonding π*-orbital on the carbonyl carbon. Of course, the UV energyirradiated may be of any suitable wavelength, although when theabove-described materials are used, a wavelength of between about320-380 nm generally is sufficient to cause the desired photochemicalattachment.

A more detailed description of example techniques for anchoring thetemplate to a substrate will now be provided. In particular, adescription of example techniques for anchoring chlorosilanebenzophenone (CSBP) to an SiO₂ surface will now be provided. CSBP may bereadily attached to SiO₂ surface or any glass surfaces using thefollowing and/or similar example techniques. In one example, CSBP wasdiluted in toluene, and a few drops of tri-ethyl amine added. The latterhelps to bind the resulting HCL and may also catalyze the reaction. Thisprocess may be performed by simply immersing the glass substrates inthis mixture, by dip coating, etc. The substrates may then be rinsed inchloroform and dried in an N₂-inclusive environment. Surface energymeasurements were used to quantify the surface coverage with CSBP. Thesurface is then ready for attaching the PAH molecules.

A more detailed description of example techniques for photochemicalattachment of the PAH to the template will now be provided. As alludedto above, the photochemical attachment of the PAH to the templateinvolves the preparation of PAH itself. In certain example embodiments,this may involve a Diels-Alder cyclo-addition, e.g., to synthesize thePAH molecules. The finished C96 PAH was purified via columnchromatography. Solutions of various concentrations of C96 were preparedin chloroform or dodecane. Of course, it will be appreciated thatpre-mixed or pre-formed PAH-inclusive solutions may be provided inconnection with certain example embodiments.

Example details regarding hexabenzocoronene (HBC) synthesis will now beprovided. As alluded to above, the PAH may be manufactured from avariety of techniques namely by a Diels-Alder cyclo-addition as theinitial step, followed by cyclo-dehydrogenation of PAH precursor formed.In example instance, a lipophilic cyclopentadienone was coupled with ahydrophilic diphenylacetelyne by the Diels-Alder reaction, and the finalproduct of that reaction was subjected to oxidative cyclization to giveHBC.

Example details regarding C96-C12 preparation will now be provided. Inone example instance, 1,3,5 triethylbenzene and 3,4-bis(4-dodecylphenyl)-2-5diphenylcyclopentadienone were dissolved in xyleneand heated for 15-20 hours at 170 degrees C. under an inert atmosphere.The solvent was removed in vacuo, and the residue was purified bychromatography on silcal gel (ether/dichloromethane) to yield C96-C12.The C96 precursor was then dissolved in dichloromethane, and FeCl₃ wasadded drop-wise. Argon was bubbled through the solution to remove anyHCL formed. The final product was about 60% pure C96 which, as notedabove, was purified by column chromatography.

Once prepared, the PAH is ready for deposition and photo-attachment tothe substrate. In this regard, a thin layer of C96 was created by spincasting and spin coat the CSBP-modified surface of the substrate.Various spin rate or dip coatings rates were used (and optimal rates maybe derived experimentally in certain example embodiments depending, forexample, on the PAH precursor, the template, the substrate, thedeposition environment, etc.), and the solvent was then allowed toevaporate. The samples were then placed on a hot plate for 1 minute toremove any additional or extraneous solvent. The surface was theirradiated with UV at 365 nm. The UV promotes an n to π* transition inthe benzophenone, which allows a reaction in the alkyl chain at C96.Following irradiation, the excess molecules were washed off. C96solutions in dodecane (or choloroform) with concentration of 10-2 to10-6 were prepared. The films prepared from dodecane resulted in veryflat PAH, which were positionally flat on the substrate. It is surmisedthat this result relates to the interaction between the solvents and themolecules as long as the alkyl chains of the solvent perturbs the π-πstart interaction of the PAH molecules and forces them to stay flat tothe surface. Another possibility is that dodecane wets better thanchloroform.

It will be appreciated that molecules with different numbers of carbonatoms and/or aromatic sextets may be used in connection with differentexample embodiments of this invention. In such cases, the details of thephotochemical attachment may vary, e.g., in terms of the wavelengthand/or energy required. FIG. 18, for example, illustrates the variationof LUMO-HOMO energy difference for carbon molecules and PAH moleculeswith varying numbers, N, of hexagonal carbons or sextets. As is known,HOMO and LUMO are acronyms for highest occupied molecular orbital andlowest unoccupied molecular orbital, respectively. The difference of theenergies of the HOMO and LUMO is termed the band gap, which sometimesmay serve as a measure of the excitability of the molecule.

A description of how heat may be used to create graphene in certainexample embodiments will now be provided. One example heat source thatmay be used is a moly-silicide heater plate (e.g., 4 inches in diameter)placed within a vacuum chamber controlled using a LABVIEW mxi DAC. Themaximum temperature achievable is 1250 degrees C., and the rate oftemperature rise may be controlled from 5 degrees C./min. to 50 degreesC./min. In some instances, the onset of pyrolysis has been observed tooccur at around 550 degrees C., and the formation of graphene has beenachieved just above 700 degrees C.

In three sets of experiments, graphene films were grown under Ar, He,and C₂H₂ gas at 0.5 Torr. In one set of experiments, the films wereheated in at atmosphere of inert gas from room temperature at a rate of50 degrees C./min. to reach a temperature as high as 600 degrees C.Graphene films readily forms, and Raman results (e.g., shown in FIG. 19below) suggest that the onset for the formation graphene starts ataround 450 degrees C. or 475 degrees C. in an atmosphere of Ar.

In the latter case, the C₂H₂ gas seems to be beneficial in “repairing”the graphene layer (or at least improving the quality of the graphenelayer) and providing a lower sheet resistance. For example, it has beendiscovered that if heating is performed under an atmosphere comprisingboth Ar and C₂H₂, then the electrocal characteristics and the quality ofthe films surprisingly and unexpectedly improve. No catalyst was usedduring this example growth process.

In terms of pyrolysis, in one example using argon-assisted thermalreduction, the substrate was first heated at 100 degrees C. for 2 hours.It was then heated to or at a selected elevated temperature (which mayrange from about 600-1000 degrees C.) for 30 min., with a rate oftemperature increase of 2 degrees C. min−1 under Ar atmosphere with aflow rate of 100 sccm. In one example using acetylene-assisted thermalreduction which, as noted above, may help with repair, the substrate wasfirst heated at 100 degrees C. for 2 h and then to or at the selectedtemperature (which may range from about 600-1000 degrees C.) for 30 min.with a rate of temperature increase of 2 to 30 degrees C. min−1 under Aratmosphere with a flow rate of 100 sccm. During the heating process, theC₂H₂ and Ar gasses were flowed for 10 min with a flow rate of 30 sccmand 100 sccm, respectively.

In any event, FIG. 19 shows the Raman Spectra and the G′ peak of thegraphene films grown in accordance with certain example embodiments.However, it will be appreciated that certain example embodiments may usecatalysts and/or dopants to further alter film properties. As is known,G and D′ peaks may be used to show whether formed films are indeedgraphene. The G and D′ relates to the orderly honeycomb structure of C—Csp2 hybridized form. The sharpness of the peaks (with a FWHM of about 25cm−1 centered at 2685 cm−1) also relates to highness in quality. Thereduced D (1350 cm−1) peak, on the other hand, provides evidence thatsp² related carbon defects is well below the detection limit of theJobin-Yvon Raman spectrometer. The D bandmap (integrated over 1300 to1400 cm−1), which is very sensitive to defects and wrinkles show a veryclear, crease or defect free region. The G′ peak may be used as themarker for the number of graphene layers and find films, where n can beas large as 4, though some shift in the G′ line from one area to theother is observable.

Although certain example embodiments have been described as involving aliquid-based precursor, different example embodiments may includegaseous precursors. For instance, in certain example embodiments, agaseous stream comprising a carrier gas and a precursor molecule (whichmay be a PAH and/or discotic molecule such as one of theabove-identified molecules) may be provided proximate to a substrate tobe coated, e.g., with the monolayer template having already beendisposed thereon. The precursor molecule may be attached to thetemplate, e.g., via UV irradiation. It will be appreciated that it maybe relatively easy to vaporize the molecules from a low pressure gaseousstream. Indeed, the inventor of the instant application has discoveredthat

HBC can be deposited using sublimation. The molecule is large enough toadd functional molecular side groups, making these classes of moleculesvery interesting for applications. Self-organization ofhexa-alkyl-substituted derivates of HBC into a columnar mesophase inorganic solvents has been demonstrated, leading to one-dimensionalconductors with a very high charge carrier mobility (e.g., molecularnanowires).

In one example instance, HBC molecules were evaporated in vacuum using aKnudsen cell at a temperature of about 620 K. The molecules then coatedthe substrate and were further pyrolized at a temperature of about 600degrees C. This device allows the effusion of HBC molecules in acontrolled fashion leading to the formation of n-layer of graphene thatcan be adsorbed over a substrate which is coated with, for example, aMoS₂ thin film. The substrate preferably will be very clean prior to HBCmolecule deposition. It will be appreciated that e-beam evaporation alsomay be used in certain example embodiments.

Perhaps more generally, however, at the sublimation temperature of 620K, the flux increases by about one order of magnitude with a temperatureincrease of 10 K. The size of the molecules that can be evaporated bymolecular beam deposition from a hot crucible typically is limited. Ingeneral, the sublimation temperature of molecules, which are onlyloosely bound by Van der Waals forces, increases with the molecularweight. The HBC molecule apparently is an exception to this generalrule, as HBC molecules grow in layers like graphite. The pi-piinteraction is incremental to the standard Van der Waals dispersive,thus increasing the sublimation temperature between the molecularplanes. If the temperature needed for sublimation of the molecule ishigher than the temperature at which the inner molecular bondsdissociate, molecules are evaporated in fragments. However, as alludedto above, this apparently does not occur with HBC and its ampiphilicderivatives.

It will be appreciated that certain example embodiments use an anchoringagent that is bonded to the glass and that the PAH attached to upon UVirradiation. For instance, di-chloro-silane benzophenone (CSBP) may beused as one such anchoring agent in certain example embodiments. Ingeneral, the anchoring agent may be attached to a glass, silica, metal,plastic, or other substrate, which may be cleaned and/or coated, forexample, with a suitable underlayer to accommodate the anchoring agent.The PAH may be mixed with a solvent. For instance, in certain exampleimplementations, HBC-C12 or C96-C12 may be dissolved in dodecane at aconcentration of 1.5-3 mg ml−1. When dodecane is used as a solvent, itmay be provided at a molar concentration of, for example, 10-6, 10-5, oreven 10-3. In any event, when wet techniques are to be used, thesolution may be spin coated, dip coated, roll coated, curtain coated,sprayed, or otherwise provided to a borosilicate glass, silicon wafer,quartz, or other substrate. Of course, a PAH precursor may be introducedvia a gaseous stream and then may be evaporated, sublimed, or otherwiseprovided to the surface of the substrate. Photoattachment, e.g., via UVirradiation may be used to attach the PAH to the anchoring agent inselect areas and/or according to a pattern. The PAH may be pyrolyzed,e.g., so as to provide for graphene growth, one layer at a time, whichmay occur in an inert (e.g., Ar, Ne, or other suitable gas) environment,with or without acetylene or other hydrocarbon gas. In any event, thistechnique leverages the inventor's recognition of those templates thatnatural have an alignment (e.g., by virtue of their thermodynamicproperties) suitable for accommodating graphene growth, as well as theinventor's recognition that certain PAH molecules already include thegraphene template. The techniques described herein are provided so thatPAH molecules may be put down in a “patchwork” in certain exampleembodiments, enabling graphene to be grown across large areas, e.g., inareas larger than individual PAH molecules.

Of course, if these and/or other similar techniques are performed in avacuum, for example, the need for a CSBP or other anchoring template maybe reduced. Thus, it will be appreciated that not all embodiments ofthis invention require an anchoring template such as CSBP.

It will be appreciated that the example techniques described herein maybe used to provide for layer-by layer growth of graphene, directly orindirectly, on substrates (e.g., glass substrates, silicon wafers,and/or the like). In other words, the example techniques describedherein may be used to provide controlled N growth of graphene-basedlayers, e.g., through repeated process steps.

The example techniques described herein may be used to produce patternedgraphene-based layers. For example, the SAM template may be provided tothe glass substrate according to a desired pattern. This may befacilitated, for example, through the use of masks, selective removal ofthe SAM template, etc. In addition, or in the alternative, thephotochemical activation (e.g., the irradiation of UV light) may becontrolled so as to only cause the adhesion of the PAH molecules to theSAM template is a predefined pattern, e.g., by restricting the areaswhere the UV light is shined through control of the UV light source,through appropriate masks (e.g., using a suitable photoresist), and/orthe like. Etching also may be used to help pattern the graphene-basedlayers, e.g., by removing portions of the SAM template, removingportions of the resulting graphene-based layer, etc. It will beappreciated that such techniques may allow electronic devices such as,for example, transistors, to be built.

As alluded to above, the search for novel electrode materials with goodstability, high transparency, and excellent conductivity is ongoing,with a crucial goal to look at alternatives to TCO's. Indium tin oxide(ITO) and fluorine tin oxide (FTO) coatings are widely used as windowelectrodes in opto-electronic devices. Despite being immenselysuccessful, these TCO's appear to be increasingly problematic because ofthe the limited availability of the element indium on earth, instabilityin the presence of acid or base, their susceptibility to ion diffusionfrom ion conducting layers, their limited transparency in the nearinfrared region (e.g., the power-rich spectrum), high leakage current ofFTO devices caused by FTO structure defects, etc. Thus, the section thatfollows identifies several example graphene-inclusive applicationswhere, for instance, TCO's such as ITO, FTO, and/or the like, may bereplaced by or supplemented with graphene-based layers.

Example Graphene-Inclusive Applications

As alluded to above, graphene-based layers may be used in a wide varietyof applications and/or electronic devices. In such example applicationsand/or electronic devices, ITO and/or other conductive layers simply maybe replaced by graphene-based layers. Making devices with graphene willtypically involve making contacts with metals, degenerate semiconductorslike ITO, solar cell semiconductors such as a-Si and CdTe among others,and/or the like.

Despite having a zero band-gap and a vanishing density of states (DOS)at the K-points in the Brillouin zone, free standing graphene exhibitsmetallic behavior. However, adsorption on metallic, semiconducting orinsulating substrate can alter its electronic properties. To compensatefor this, additionally, or in the alternative, in example applicationsand/or electronic devices, the graphene-based layer may be doped inaccordance with any semiconductor layers adjacent thereto. That is, incertain example embodiments, if a graphene-based layer is adjacent to ann-type semiconductor layer, the graphene-based layer may be doped withan n-type dopant. Likewise, in certain example embodiments, if agraphene-based layer is adjacent to a p-type semiconductor layer, thegraphene-based layer may be doped with a p-type dopant. Of course, theshift in Fermi level in graphene with respect to the conical points maybe modeled, e.g., using density functional theory (DFT). Band-gapcalculations show that metal/graphene interfaces can be classified intotwo broad classes, namely, chemisorption and physisorption. In thelatter case, a shift upward (downward) means that electron (holes) aredonated by the metal to the graphene. Thus, it is possible to predictwhich metal or TCO to use to as contacts to the graphene depending onthe application.

A first example electronic device that may make use of one or moregraphene-based layers is a solar photovoltaic device. Such exampledevices may include front electrodes or back electrodes. In suchdevices, the graphene-based layers may simply replace the ITO typicallyused therein. Photovoltaic devices are disclosed in, for example, U.S.Pat. Nos. 6,784,361, 6,288,325, 6,613,603 and 6,123,824; U.S.Publication Nos. 2008/0169021; 2009/0032098; 2008/0308147; and2009/0020157; and application Ser. Nos. 12/285,374, 12/285,890, and12/457,006, the disclosures of which are hereby incorporated herein byreference.

Alternatively, or in addition, doped graphene-based layers may beincluded therein so as to match with adjacent semiconductor layers. Forinstance, FIG. 12 is a cross-sectional schematic view of a solarphotovoltaic device incorporating graphene-based layers according tocertain example embodiments. In the FIG. 12 example embodiment, a glasssubstrate 1202 is provided. For example and without limitation, theglass substrate 1202 may be of any of the glasses described in any ofU.S. patent application Ser. Nos. 11/049,292 and/or 11/122,218, thedisclosures of which are hereby incorporated herein by reference. Theglass substrate optionally may be nano-textured, e.g., to increase theefficiency of the solar cell. An anti-reflective (AR) coating 1204 maybe provided on an exterior surface of the glass substrate 1202, e.g., toincrease transmission. The anti-reflective coating 1204 may be asingle-layer anti-reflective (SLAR) coating (e.g., a silicon oxideanti-reflective coating) or a multi-layer anti-reflective (MLAR)coating. Such AR coatings may be provided using any suitable technique.

One or more absorbing layers 1206 may be provided on the glass substrate1202 opposite the AR coating 1204, e.g., in the case of a back electrodedevice such as that shown in the FIG. 12 example embodiment. Theabsorbing layers 1206 may be sandwiched between first and secondsemi-conductors. In the FIG. 12 example embodiment, absorbing layers1206 are sandwiched between n-type semiconductor layer 1208 (closer tothe glass substrate 1202) and p-type semiconductor 1210 (farther fromthe glass substrate 1202). A back contact 1212 (e.g., of aluminum orother suitable material) also may be provided. Rather than providing ITOor other conductive material(s) between the semiconductor 1208 and theglass substrate 1202 and/or between the semiconductor 1210 and the backcontact 1212, first and second graphene-based layers 1214 and 1216 maybe provided. The graphene-based layers 1214 and 1216 may be doped so asto match the adjacent semiconductor layers 1208 and 1210, respectively.Thus, in the FIG. 12 example embodiment, graphene-based layer 1214 maybe doped with n-type dopants and graphene-based layer 1216 may be dopedwith p-type dopants.

Because it is difficult to directly texture graphene, an optional layer1218 may be provided between the glass substrate 1202 and the firstgraphene-based layer 1214. However, because graphene is very flexible,it generally will conform to the surface on which it is placed.Accordingly, it is possible to texture the optional layer 1218 so thatthe texture of that layer may be “transferred” or otherwise reflected inthe generally conformal graphene-based layer 1214. In this regard, theoptional textured layer 1218 may comprise zinc-doped tin oxide (ZTO). Itis noted that one or both of semiconductors 1208 and 1210 may bereplaced with polymeric conductive materials in certain exampleembodiments.

Because graphene is essentially transparent in the near and mid-IRranges implies that the most penetrating long wavelength radiation maypenetrate and generate carriers deep into the i-layer of both single andtandem junction solar cells. This implies that the need to texture backcontacts may not be needed with graphene-based layers, as the efficiencywill already be increased by as much as several percentage points.

Screen-printing, evaporation, and sintering technologies and CdCl2treatment at high temperatures are currently used in CdS/CdTe solar cellheterojunctions. These cells have high fill factors (FF>0.8). However,series resistance Rs is an efficiency limiting artifact. In Rs, there isa distributed part from sheet resistance of the CdS layer and a discretecomponent associated with the CdTe and graphite based contact on top ofit. The use of one or more graphene-based layers may help reduce bothcontributions to Rs, while preserving good heterojunction properties. Byincluding graphene in such a solar structure for both front and backcontact arrangements, a substantial efficiency boost may be achieved.

It will be appreciated that certain example embodiments may involvesingle-junction solar cells, whereas certain example embodiments mayinvolve tandem solar cells. Certain example embodiments may be CdS,CdTe, CIS/CIGS, a-Si, and/or other types of solar cells.

Another example embodiment that may incorporate one or moregraphene-based layers is a touch panel display. For instance, the touchpanel display may be a capacitive or resistive touch panel displayincluding ITO or other conductive layers. See, for example, U.S. Pat.Nos. 7,436,393; 7,372,510; 7,215,331; 6,204,897; 6,177,918; and5,650,597, and application Ser. No. 12/292,406, the disclosures of whichare hereby incorporated herein by reference. The ITO and/or otherconductive layers may be replaced in such touch panels may be replacedwith graphene-based layers. For example, FIG. 13 is a cross-sectionalschematic view of a touch screen incorporating graphene-based layersaccording to certain example embodiments. FIG. 13 includes an underlyingdisplay 1302, which may, in certain example embodiments, be an LCD,plasma, or other flat panel display. An optically clear adhesive 1304couples the display 1302 to a thin glass sheet 1306. A deformable PETfoil 1308 is provided as the top-most layer in the FIG. 13 exampleembodiment. The PET foil 1308 is spaced apart from the upper surface ofthe thin glass substrate 1306 by virtual of a plurality of pillarspacers 1310 and edge seals 1312. First and second graphene-based layers1314 and 1316 may be provided on the surface of the PET foil 1308 closerto the display 1302 and to the thin glass substrate 1306 on the surfacefacing the PET foil 1308, respectively. One or both graphene-basedlayers 1314 and 1316 may be patterned, e.g., by ion beam and/or laseretching. It is noted that the graphene-based layer on the PET foil maybe transferred from its growth location to the intermediate productusing the PET foil itself. In other words, the PET foil may be usedinstead of a photoresist or other material when lifting off the grapheneand/or moving it.

A sheet resistance of less than about 500 ohms/square for thegraphene-based layers is acceptable in embodiments similar to thoseshown in FIG. 13, and a sheet resistance of less than about 300ohms/square is advantageous for the graphene-based layers.

It will be appreciated that the ITO typically found in display 1302 maybe replaced with one or more graphene-based layers. For example, whendisplay 1302 is an LCD display, graphene-based layers may be provided asa common electrode on the color filter substrate and/or as patternedelectrodes on the so-called TFT substrate. Of course, graphene-basedlayers, doped or undoped, also may be used in connection with the designand fabrication of the individual TFTs. Similar arrangements also may beprovided in connection with plasma and/or other plat panel displays.

Graphene-based layers also may be used to create conductive data/buslines, bus bars, antennas, and/or the like. Such structures may beformed on/applied to glass substrates, silicon wafers, etc. FIG. 14 is aflowchart illustrating an example technique for forming a conductivedata/bus line in accordance with certain example embodiments. In stepS1401, a graphene-based layer is formed on an appropriate substrate. Inan optional step, step S1403, a protective layer may be provided overthe graphene-based layer. In step S1405, the graphene-based layer isselectively removed or patterned. This removal or patterning may beaccomplished by laser etching. In such cases, the need for a protectivelayer may be reduced, provided that the resolution of the laser is fineenough. Alternatively or in addition, etching may be performed viaexposure to an ion beam/plasma treatment. Also, as explained above, H*may be used, e.g., in connection with a hot filament. When an ionbeam/plasma treatment is used for etching, a protective layer may bedesirable. For example, a photoresist material may be used to protectthe graphene areas of interest. Such a photoresist may be applied, e.g.,by spin coating or the like in step S1403. In such cases, in anotheroptional step, S1407, the optional protective layer is removed. Exposureto UV radiation may be used with appropriate photoresists, for example.In one or more steps not shown, the conductive graphene-based patternmay be transferred to an intermediate or final product if it was notalready formed thereon, e.g., using any appropriate technique (such as,for example, those described above).

Although certain example embodiments have been described as etching awayor removing graphene-based layers, certain example embodiments maysimply change the conductivity of the graphene-based layer. In suchcases, some or all of the graphene may not be removed. However, becausethe conductivity has been suitably altered, only the appropriatelypatterned areas may be conductive.

FIG. 15 is a schematic view of a technique for forming a conductivedata/bus line in accordance with certain example embodiments. As shownin FIG. 15, the conductivity of the graphene is selectively changed byvirtue of exposure to an ion beam. A photoresist is applied in asuitable pattern, e.g., so as to protect desired portions of thegraphene-based layer, whereas the other portions of the graphene-basedlayer remain exposed to the ion beam/plasma.

Mobility data is shown in the table below after various samples havebeen deposited and etched.

Etched Thickness Rho Conductivty Mobility μ Samples Passes (Å) (Ωcm)(1/Ωcm) (cm{circumflex over ( )}2/Vs) A 25 8 1.03E−04 970000 120,000 B20 6 5.24E−03 1010000 143000 C 10 6 5.94E−02 1600000 150000 D 5 61.48E−02 1500000 160000

It will be appreciated that patterning the graphene in this and/or otherways may be advantageous for a number of reasons. For example, the layerwill be largely transparent. Thus, it is possible to provide “seamless”antennas where the pattern cannot be seen. A similar result may beprovided in connection with bus bars that may be incorporated intovehicle windows (e.g., for defrosting, antenna usage, poweringcomponents, etc.), flat panel (e.g., LCD, plasma, and/or other) displaydevices, skylights, refrigerator/freezer doors/windows, etc. This mayalso advantageously reduce the need for black frits often found in suchproducts. Additionally, graphene-based layers may be used in place ofITO in electrochromic devices.

Although certain example applications/devices have been describedherein, as shown above, it is possible to use graphene-based conductivelayers in place of or in addition to other transparent conductivecoatings (TCCs), such as ITO, zinc oxide, etc.

As used herein, the terms “on,” “supported by,” and the like should notbe interpreted to mean that two elements are directly adjacent to oneanother unless explicitly stated. In other words, a first layer may besaid to be “on” or “supported by” a second layer, even if there are oneor more layers therebetween.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A method of making a coated article, the method comprising: providinga substrate having a surface to be coated; disposing a self-assembledmonolayer (SAM) template on the surface to be coated; providing aprecursor comprising a precursor molecule, the precursor molecule beinga polycyclic aromatic hydrocarbon (PAH) and discotic molecule;dissolving the precursor to form a solution; applying the solution tothe substrate having the SAM template disposed thereon; photochemicallyattaching the precursor molecule to the SAM template; and heating thesubstrate to at least 450 degrees C. to form a graphene-inclusive film.2. The method of claim 1, wherein the substrate is a glass substrate. 3.The method of claim 1, wherein the substrate is a silicon wafer.
 4. Themethod of claim 1, further comprising providing a layer comprisingsilicon oxide on the surface to be coated prior to the disposing of theSAM template.
 5. The method of claim 1, wherein the SAM template is asilane-benzophenone derivative.
 6. The method of claim 5, wherein theSAM template is chlorosilane benzophenone (CSBP).
 7. The method of claim1, wherein the SAM template and/or the precursor molecule comprise(s)one or more alkyl groups.
 8. The method of claim 1, wherein theprecursor molecule is C96 and/or HBC.
 9. The method of claim 1, furthercomprising purifying the solution via column chromatography.
 10. Themethod of claim 9, further comprising preparing the solution inchloroform and/or dodecane.
 11. The method of claim 1, wherein thec-axis of precursor molecule is substantially perpendicular to substrateprior to and/or following the photochemical attaching.
 12. The method ofclaim 1, wherein the photochemical attaching includes irradiating UVenergy towards the substrate.
 13. The method of claim 12, wherein thewavelength of the UV energy is between 320-380 nm.
 14. The method ofclaim 1, wherein said heating is performed in a vacuum.
 15. The methodof claim 1, wherein said heating is performed in an environmentcomprising inert gas.
 16. The method of claim 15, wherein said heatingis performed in an environment comprising Ar and C₂H₂ gasses.
 17. Amethod of making a coated article, the method comprising: providing asubstrate having a surface to be coated; disposing a self-assembledmonolayer (SAM) template on the surface to be coated; applying asolution to the substrate having the SAM template disposed thereon, thesolution comprising a precursor including a precursor molecule, theprecursor molecule being a polycyclic aromatic hydrocarbon (PAH)molecule; attaching the precursor molecule to the SAM template byirradiating UV energy thereon; and heating the substrate to at least 450degrees C. to form a graphene-inclusive film, wherein the SAM templateand/or the precursor molecule comprise(s) one or more alkyl groups tohelp ensure that the c-axis of precursor molecule is substantiallyperpendicular to substrate prior to and/or following the photochemicalattaching.
 18. The method of claim 17, further comprising providing alayer comprising silicon oxide on the surface to be coated prior to thedisposing of the SAM template.
 19. The method of claim 17, wherein theSAM template is a silane-benzophenone derivative.
 20. The method ofclaim 17, wherein the precursor molecule is C96 and/or HBC.
 21. Themethod of claim 17, wherein said heating is performed in a vacuum. 22.The method of claim 17, wherein said heating is performed in anenvironment comprising inert gas.
 23. A method of making an electronicdevice, the method comprising: providing a substrate having a surface tobe coated; disposing a self-assembled monolayer (SAM) template on thesurface to be coated; providing a precursor comprising a precursormolecule, the precursor molecule being a polycyclic aromatic hydrocarbon(PAH) and discotic molecule; dissolving the precursor to form asolution; applying the solution to the substrate having the SAM templatedisposed thereon; photochemically attaching the precursor molecule tothe SAM template; heating the substrate to at least 450 degrees C. toform a graphene-inclusive film on the substrate; and building thesubstrate with the graphene-inclusive film into the electronic device.24. A method of making an electronic device, the method comprising:providing a substrate having a surface to be coated; disposing aself-assembled monolayer (SAM) template on the surface to be coated;applying a solution to the substrate having the SAM template disposedthereon, the solution comprising a precursor including a precursormolecule, the precursor molecule being a polycyclic aromatic hydrocarbon(PAH) molecule; attaching the precursor molecule to the SAM template byirradiating UV energy thereon; heating the substrate to at least 450degrees C. to form a graphene-inclusive film; and building the substratewith the graphene-inclusive film into the electronic device, wherein theSAM template and/or the precursor molecule comprise(s) one or more alkylgroups to help ensure that the c-axis of precursor molecule issubstantially perpendicular to substrate prior to and/or following thephotochemical attaching.
 25. A method of making a coated article, themethod comprising: providing a substrate having a surface to be coated;disposing a monolayer template on the surface to be coated; providing agaseous stream comprising a carrier gas and a precursor moleculeproximate to the substrate having the monolayer template disposedthereon, the precursor molecule being a polycyclic aromatic hydrocarbon(PAH) molecule; attaching the precursor molecule to the monolayertemplate by irradiating UV energy thereon; and heating the substratewith the monolayer template and the precursor molecule to form agraphene-inclusive film, wherein the monolayer template and/or theprecursor molecule comprise(s) one or more alkyl groups to help ensurethat the c-axis of precursor molecule is substantially perpendicular tosubstrate prior to and/or following the photochemical attaching.